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The know-it-best people
Even old and well tested code will be questioned by the know-it-best people.
The opposite of the previous paragraph. No matter how well tested and proven a particular functionality or piece of code is, every now and then the I-know-better-than-everyone-else person shows up and tells the community how things really are and how to do things to work correctly. Then it does not matter if you object, because the newcomer obviously is one of the smartest persons on the planet. When it happens (when, not if), just stay calm, polite and use reason and arguments. If that fails, my personal way is usually to slowly get involved as little as possible in such conversations.